The invention relates to the use of surfactants for lysing liposomes, particularly for liposomes that have high transition temperatures between 35.degree. C. and 65.degree. C.
Liposomes are frequently used as drug delivery vehicles which allow entrapped molecules to escape under various conditions and after various periods of time. In an article by M. Sila et al. entitled "Effects Of Triton X-100 Concentration And Incubation Temperature On Carboxyfluorescein Release From Multilamellar Liposomes", as published in Biochimica et Biophysica Acta 859 (1986), pp. 165-170, the lysis of various multilamellar liposomes with Triton-X is described; Carboxyfluorescein is a fluorescent dye which is commonly used as a marker to determine the rates at which water-soluble substances leak from liposomes. The three "hard" liposomes discussed in the article contained saturated phospholipids. Specifically, distearoyl-L-.alpha.-phosphatidylcholine-/cholesterol (2:1 mole ratio), dipalmitoyl-L-.alpha.-phosphatidylcholine and L-.alpha.phosphatidylcholine were tested. The carboxyfluorescein was excited by a spectrofluorometer at 490 nm and the emitted light read at 520 nm. The experiments showed that the use of Rohm and Haas' Triton X-100.TM. (polyethyleneglycol (9-10) p-t-octylphenol) as a lysing agent did not result in the instantaneous destabilization of the liposome and release of the marker. The amount and rate of release were found to be dependent on the lipid composition of the liposome, the concentration of the Triton X-100, and the temperature and duration of incubation.
In contrast, U.S. Pat. No. 4,707,441 is directed to the use of liposome-compatible surfactants in connection with "soft" liposomes, i.e. those made from egg lecithin, containing unsaturated phospholipids. A number of surfactants, including those sold under the trade names Igepal (GAF Corp.) and Triton (Rohm and Haas), were tested. Some of them were found to lyse liposomes and were therefore unsuitable for the purposes described in the patent.
U.S. Pat. No. 4,713,324 discloses the lysis of liposomes by detergents or by immunological reaction. The liposome may contain one of a variety of markers such as tempocholine, a fluor and a quencher, or potassium ions. In one test, the serum levels of theophylline, a drug used to treat bronchial asthma, was determined. The "soft" liposomes for this test were formed from egg lecithin. The marker used was a chemiluminescent compound in tris (hydroxymethyl) aminomethane buffer. Triton X-100 was used to lyse the liposomes. The luminescence of intact and lysed liposomes were measured and analyzed.
While Triton X-100 was found to be effective for lysing some liposomes, particularly those having relatively low transition temperatures, it is only marginally effective upon more difficult-to-lyse liposomes, particularly those containing phospholipids with relatively high transition temperatures (e.g. stearoyl phosphatides). Sodium deoxycholate and sodium dodecylsulfate (SDS) are two other compounds frequently employed to effect lysis.